1. Field of the Invention
The present invention generally relates to a structure of an electric generator incorporated into a hybrid vehicle. More particularly, the present invention relates to a cooling system for cooling the electric generator in the hybrid vehicle.
2. Description of the Related Art
In an exemplary hybrid vehicle including an engine and a drive motor as driving power sources, a water-cooling system includes a radiator for cooling the engine, an electric generator, an electric motor, an inverter, and other components. For example, Japanese Laid-Open Patent Application Publication No. 2002-4860 discloses a water-cooling system in which the electric generator, the electric motor, and the inverter are provided with cooling jackets, respectively and cooling water is circulated between the radiator and the water jackets. In this water-cooling system, convection heat transfer of a liquid phase is performed between the cooling water, and the electric generator and the electric motor which have relatively lower heat generation densities, thereby transferring and removing heat from the electric generator and the electric motor, while boiling heat transfer is performed between the cooling water and the inverter having a higher heat generation density, thereby transferring and removing heat from the inverter.
Among hybrid vehicles, in a series-hybrid vehicle in which an engine is mechanically separated from drive wheels, an electric generator can be actuated while maintaining the engine having a relatively low driving power output, in a high-efficiency running range. For this reason, in vehicles such as a golf cart or a farming truck, which do not require a high mobility, heat generation could not be problematic.
In the conventional water-cooling system, a radiator, cooling water pipes, and a cooling water pump are required, and a total weight of these components and the cooling water tends to be great. These components increase costs and are difficult to lay out.
As a solution to this, a forcible air-cooling system including a fan operative according to the operation of a rotor of the electric generator or the electric motor may possibly be used. However, under a low-load state, the electric generator does not require forcible air-cooling. In such a situation, air friction resistance generated by the rotation of the fan will result in a driving loss. Moreover, the fan increases costs.